1. Field of the Invention
The present invention relates to an optical disk. More specifically, the present invention relates to an optical disk in which a second recording layer which reflects a second wavelength laser beam is laminated onto an opposite side of a first recording layer which reflects a first wavelength laser beam, and a method of manufacturing thereof.
2. Description of the Prior Art
There is a hybrid optical disk capable of reproducing signals having a plurality of standards such as a CD (Compact Disk), a DVD (Digital Versatile Disk), and etc. from a single optical disk. Examples of such the optical disk are disclosed in Japanese Patent Laying-open No. 11-16209 [G11B 7/24] laid-open on Jan. 22, 1999 and in Japanese Patent Laying-open No. 2000-331381 [G11B 7/24] laid-open on Nov. 30, 2000.
These optical disks have structure that a first substrate which records a first signal such as a CD signal, for example and a second substrate which records a second signal such as a DVD signal, for example are pasted or laminated to each other. Then, provided that the second substrate be an irradiation side of a laser beam, the first signal recorded on the first substrate is reproduced based on a laser beam that transmits the second substrate and is reflected by pits formed on the first substrate, and the second signal recorded on the second substrate is reproduced based on a laser beam reflected by pits formed on the second substrate. Therefore, a layer between the first substrate and the second substrate (hereinafter referred to as “intermediate layer”) needs to transmit the first laser beam for reproducing the first signal and reflect the second laser beam for reproducing the second signal. It is noted that the first laser beam and the second laser beam are different in wavelength.
The intermediate layer needs to transmit the first laser beam for reproducing the first signal, and has an optical transparency so that, in general, a reflectivity of the second laser beam for reproducing the second light at the intermediate layer becomes low. Accordingly, there are problems that it needs to enhance an output of a laser beam to be irradiated and that it needs to amplify a laser beam signal being received.
However, if the output of the laser beam is enhanced, there arises a problem that not only a cost of an optical pickup becomes expensive, but also a durability of the laser is shortened.